56 ELEMENTARY BIOLOGY. [VIT. 



united. Inclosed between these narrow, dark-brown bands 

 are, usually, two elongated, oval, yellowish-brown bands; 

 and outside them, lie a number of similarly coloured patches, 

 one of which is usually considerably longer than the others. 



A longitudinal section shews that each of these patches 

 of colour answers to the transverse section of a band of 

 similar substance, which extends throughout the whole length 

 of the stem ; sometimes remaining distinct, sometimes giving 

 off branches which run into adjacent bands, and sometimes 

 uniting altogether with them. 



At a short distance below the apex of the stem, however, 

 the colour of all the bands fades away, and they are traceable 

 into mere streaks, which finally disappear altogether in the 

 semi-transparent gelatinous substance which forms the grow- 

 ing end of the stem. Submitted to microscopic examination, 

 the white ground substance, or parenchyma, is seen to consist 

 of large polygonal cells, containing numerous starch granules ; 

 and the circumferential zone is formed of somewhat elongated 

 cells, the thick walls of which have acquired a dark-brown 

 colour, and contain little or no starch. The dark-brown bands, 

 on the other hand, consist of cells which are so much elongated 

 as almost to deserve the name of fibres and constitute what is 

 termed sderenchyma. Their walls are very thick, and of 

 a deep-brown colour; but the thickening has taken place 

 unequally, so as to leave short, obliquely directed, thin places, 

 which look like clefts. The yellow bands, lastly, are vascular 

 bundles. Each consists, externally, of thick-walled, elongated, 

 parallel-sided cells, internal to which lie elongated tubes 

 devoid of protoplasm, and frequently containing air. In the 

 majority of these tubes, and in all the widest, the walls are 

 greatly thickened, the thickening having taken place along 

 equidistant transverse lines. The tubes have become flat- 

 tened against one another, by mutual pressure, so that they 

 are five- or six-sided ; and, as the markings of their flattened 



